The detection and measurement of hazardous electromagnetic radiation which may pose a biological threat has become increasingly more important in view of the rapid proliferation of electronic appliances such as micro-wave ovens, marine radar systems, medical apparatus such as diathermy equipment, and the like. Recently, both state and federal agencies have promulgated criteria for maximum electromagnetic radiation exposure in the vicinity of domestic and commercial RF and micro-wave appliances. Accurate monitoring of such appliances requires that the introduction of the monitoring apparatus into the vicinity of the appliance does not result in a significant perturbation of the electromagnetic field to be measured, and a consequent loading of the radiation source. In addition, short duration bursts of radiation from devices such as radar systems must be detectable, requiring a measurement system with a relatively large bandwidth. Furthermore, the relatively small maximum permissable radiation exposure safety levels require a detector capable of measuring small field strengths. The capacity of the detector to identify the polarization of the source of prohibited electromagnetic radiation is also important.
Prior art electromagnetic field measurement devices have utilized metallic cables, which tend to perturb the field being monitored, reducing the accuracy of the resulting measurements. A more recent field measurement system employs diode detectors connected to an antenna, to detect electromagnetic radiation. The diode detector employs a high-ohmic transmission line to transfer the detected signal to a readout device which allows extraneous noise to be picked up by the antenna. The bandwidth of the information transmitted on the high-ohmic transmission line is quite small, precluding the observation of short electromagnetic pulses or rapid modulation of the fields under study. Light emitting diodes connected to an antenna are impractical for the measurement of fields whose strengths even approach the small magnitude required to enforce the maximum radiation exposure criteria. The prior art light emitting diode is an active device, drawing its power from the field being monitored. Approximately 1.3 volts must be applied to the light emitting diode before any light is emitted whatsoever. However, only a few millivolts of radio-frequency or micro-wave energy are available from an electrically small, non-perturbing antenna in the presence of an electric field whose intensity is strong enough to just exceed the radiation exposure hazard criteria for RF or micro-wave appliances. In addition, the prior art light emitting diode device distorts the instantaneous signal which is received by the antenna to which it is connected, thereby reducing the accuracy of the measurement of the amplitude, phase and frequency thereof.
A need therefore exists for an electromagnetic field measuring device which provides a long-sought solution to the problem of accurately measuring the amplitude, phase and frequency of hazardous electromagnetic radiation whose magnitude is in the range of the permissable radiation exposure criteria.
It is therefore an object of the invention to monitor electromagnetic radiation in free space or in the near field of a radiating source in an improved manner, including amplitude phase and frequency.
It is still another object of the invention to monitor electromagnetic radiation in free space or in the near field of a radiation source without introducing perturbations to the field being monitored, in an improved manner.
It is still a further object of the invention to monitor electromagnetic radiation in free space or in the near field of a radiating source which appears as a short duration burst or a rapid modulation, in an improved manner.
It is still a further object of the invention to monitor electromagnetic radiation in free space or in the near field of a radiating source so as to determine the polorization of the fields, in an improved manner.
It is yet another object of the invention to monitor electromagnetic radiation having a small field strength (either electric or magnetic fields), in an improved manner.
It is a further object of the invention to provide an improved, portable and accurate electromagnetic field measuring device having a probe of such design as to cause minimal perturbation of the field, permitting field strength measurements at a physical location close to the field source without loading the field source.
Still further objects of the invention particularly appropriate for its use as a portable measuring instrument concern the provision of an improved electromagnetic field measuring device which is stable in operation, immune to burnout, rugged, lightweight, physically small, and capable of operation by a self-contained power source.